1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor film having a crystalline structure, and to a method of manufacturing the semiconductor device. More specifically, the present invention relates to a semiconductor device having a thin film transistor (TFT) containing an active layer having a channel forming region, a source region, and a drain region from a semiconductor film having a crystalline structure, and a method of manufacturing the semiconductor device.
2. Description of the Related Art
Liquid crystal display devices have widened the marketplace with their advantages of low electric power consumption and space conservation, and have finally reached the point of substituting for CRTs as televisions used daily in households. As such, bright display at high definition equal to or better than that of CRTs, and a price comparable to that of CRTs, are sought for liquid crystal display devices.
Demanded of TFTs formed in pixel portions of liquid crystal display devices in general use as displays (display devices) is mainly a low off current (electric current flowing when the TFTs are in an off operation state). Even a slight leak of the off current in an off operation state of the TFT invites a reduction in contrast and in image quality. There has been a problem with TFTs containing active layers having a crystalline structure, which are recently used more and more due to their high field effect mobility, in that the off current becomes large.
An LDD (lightly doped drain) structure is known as a technique of suppressing the off current. In Japanese patent #3072655 (structure shown in FIG. 2A), a structure is disclosed in which a first transistor and a second transistor are connected in series, sandwiching a low concentration impurity region (double gate structure having a low concentration impurity region sandwiched by channel forming regions).
The demand for making liquid crystal display devices have higher definition is met by increasing the number of pixels, and the demand for higher brightness is met by increasing the aperture ratio. Screen size is determined by standards, and therefore it is necessary to increase the number of pixels within a limited pixel surface area. This means that the pixel size must be shrunk, and that a technique for further increasing the aperture ratio must be achieved while reducing the pixel size. There are limitations on making the wiring width narrower for increase of the aperture ratio, considering problems such as a rise of the wiring resistance. Thus, making the size of switching TFTs within the pixels smaller can be considered.
Further, the size of storage capacitors can be made smaller provided that the off current of the switching TFTs of the pixels can be made smaller. Therefore, it is very important to make TFTs having a low off current in order to additionally increase the aperture ratio.
However, the characteristics required by the circuits used on the same substrate are different. It is necessary to make TFTs having different structures depending upon which circuit they are used for. TFTs are manufactured by lamination while performing steps for etching semiconductor films, insulating films, and conductive films into predetermined shapes using photomasks. Therefore, if the TFT structure is optimized according to the requirements of the pixel portion or each driver circuit, then the number of photomasks simply increases, the manufacturing processes become complex, and the number of process steps inevitably increases. Further, TFTs having a sufficiently low off current cannot be manufactured even if small TFTs are manufactured in order to increase the aperture ratio in the pixel portion, and the reliability drops. It is not easy to manufacture the desired display device (semiconductor device).